The Ingredients for Life (Revised)

Today, NASA scientists announced that a bacterium strain called GFAJ-1 substitutes arsenic for phosphorus, a once-essential component for life. Wait, this is big. GFAJ-1 is the first organism ever found to live without phosphorous—taking one more element out of the must-have list for life to exist. Yet another surprising story of extreme survival on Earth, GFAJ-1 will help inform extraterrestrial-seeking scientists where best to look for life on other planets. Here's a look at the (now-revised) ingredients needed to make life.

What Life Needs

Raw Materials

All life on Earth is composed of the same basic basic building blocks: DNA, RNA, proteins and lipids. Protein, the structure and catalyst for biochemical reactions, is made from amino acids. Lipids, which hold cells together and store energy, are made from carbon and hydrogen. Then there's DNA and RNA, the most basic building blocks of life—and, some argue, by themselves the first forms of life—made of a sugar called ribose and of phosphates, the key player in today's discovery.

"Phosphate, part of that RNA and DNA structure, is the ultimate nutrient," according to Christopher House, Associate Professor of Geosciences at Penn State University, "It is the limiting factor for much of life on Earth." Unlike nitrogen or carbon, which can be manufactured from the air (at an energy cost to the organism), phosphorus must be present in the environment for life as we know it. Or should I say, knew it. GFAJ-1, a member of the Halomonadaceae family, breaks this rule, according to findings published in the journal Science, and shows that arsenic can take the place of phosphorus, acting as a substitute for the essential role that phosphorous provides in sustaining life.

For organisms on Earth, there's no more important ingredient for survival than water. It's not the H2O itself that is necessary for all life, but liquids in general. "There can't be life in a solid, and there can't be life in a gas," the University of Washington's Peter Ward told Carl Zimmer in a 2006 cover story for Popular Mechanics. "In a gas, molecules are flying around so quickly that they can't carry out the complicated chemical reactions necessary for life," Ward said. "In a solid, they can barely move at all."

Other liquid mediums could prove to be hosting life as well. "The most plausible would be ammonia," House says, like that found on Saturn's moon, Enceladus. Liquid methane, like that found on Titan, could also be hospitable to life.

Energy

Most of life on Earth—over 99 percent of it—depends on the sun. Whether the sustenance made from the sun is in the form of photosynthesis or solar energy received further down the food chain, our star is essential for most of life as we know it.

If you shut off photosynthesis, you could have life that thrives on geochemical energy, House says. Ultimately, without the sun, our planet would be less active, but there would be still energy coming from the original accretion of Earth—that is, the gravitational collapse of the planet—and secondarily, from the radioactive decay of elements within the mantle.

Radioactive elements can more directly sustain organisms as well. Microbiologists have found forms of life that use the hydrogen, split apart by radioactive particles from the the likes of uranium or thorium underground or in deep-sea sediments. This nuclear power is another—very rare—form of energy for life, one that, ultimately, is solar energy too—created out of a distant supernovae.